Machinery's Handbook, 31st Edition
2244 Backlash in Gears Recommended Backlash: In the following tables American Gear Manufacturers Association recommendations for backlash ranges for various kinds of gears are given. * For purposes of measurement and calculation, backlash is defined as the amount by which a tooth space exceeds the thickness of an engaging tooth. When not otherwise specified, numerical values of backlash are understood to be measured at the tightest point of mesh on the pitch circle in a direction normal to the tooth surface when the gears are mounted in their specified position. Coarse-Pitch Gears: Table 1 gives the recommended backlash range for coarse-pitch spur, helical and herringbone gearing. Because backlash for helical and herringbone gears is more conveniently measured in the normal plane, Table 1 has been prepared to show backlash in the normal plane for coarse-pitch helical and herringbone gearing and in the transverse plane for spur gears. To obtain backlash in the transverse plane for helical and herringbone gears, divide the normal plane backlash in Table 1 by the cosine of the helix angle. Table 2. AGMA Recommended Backlash Range for Bevel and Hypoid Gears
Normal Backlash, Inch
Normal Backlash, Inch
Quality Numbers 7 through 13 0.020-0.030 0.018-0.026 0.016-0.022 0.014-0.018 0.012-0.016 0.010-0.013 0.008-0.011 0.007-0.009 0.006-0.008
Quality Numbers 3 through 6 0.045-0.065 0.035-0.055 0.025-0.045 0.020-0.040 0.020-0.030 0.015-0.025 0.012-0.022 0.010-0.020 0.008-0.016
Quality Numbers 7 through 13 0.005-0.007 0.004-0.006 0.003-0.005 0.002-0.004 0.001-0.003 0.000-0.002 0.000-0.001 0.000-0.0007
Quality Numbers 3 through 6 0.006-0.013 0.005-0.010 0.004-0.008 0.003-0.005 0.002-0.004 0.000-0.002 0.000-0.001 0.000-0.0007
Diametral Pitch 1.00 to 1.25 1.25 to 1.50 1.50 to 1.75 1.75 to 2.00 2.00 to 2.50 2.50 to 3.00 3.00 to 3.50 3.50 to 4.00 4.00 to 5.00
Diametral Pitch 5.00 to 6.00 6.00 to 8.00 8.00 to 10.00 10.00 to 16.00 16.00 to 20.00
20 to 50 50 to 80
80 and finer
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…
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Measured at tightest point of mesh The backlash tolerances given in this table contain allowances for gear expansion due to a differential in the operating temperature of the gearing and their supporting structure. The values may be used where the operating temperature is up to 70 °F higher (39 °C higher) than the ambient temperature. These backlash values will provide proper running clearances for most gear applications. The following important factors must be considered in establishing backlash toler- ances: a) Center distance tolerance; b) Parallelism of gear axes; c) Side runout or wobble; d) Tooth thickness tolerance; e) Pitch line runout tolerance; f) Profile tolerance; g) Pitch tolerance; h) Lead tolerance; i) Types of bearings and subsequent wear; j) Deflection under load; k) Gear tooth wear; l) Pitch line velocity; m) Lubrication requirements; and n) Thermal expansion of gears and housing. A tight mesh may result in objectionable gear sound, increased power losses, overheat ing, rupture of the lubricant film, overloaded bearings and premature gear failure. How ever, it is recognized that there are some gearing applications where a tight mesh (zero backlash) may be required. Specifying unnecessarily close backlash tolerances will increase the cost of the gearing. It is obvious from the above summary that the desired amount of backlash is difficult to evaluate. It is, therefore, recommended that when a designer, user or purchaser includes a reference to backlash in a gearing specification and drawing, consultation be arranged with the manufacturer. * Extracted from Gear Classification Manual, AGMA 390.03 with permission of the publisher, the Amer- ican Gear Manufacturers Association, 1500 King St., Alexandria, VA 22314.
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